Certificate of correction



United States Patent 3,125,445 NIOBIUM BASE ALLOY Neil M. Lottridge, J12, Warren, Micl1., assignor to General Motors Corporation, Detroit, Mic-la, a corporation of Delaware No Drawing. Filed May 26, 1959, Ser. No. 816,009 8 Claims. (Cl. 75--174) This invention relates to a niobium base alloy having a desirable combination of high hot strength, excellent fabricability, high room temperature impact strength, outstanding hot ductility, high melting temperature and improved oxidation resistance. It pertains perticularly to a refractory alloy of this type which is designed as a material for a turbine blade or other gas turbine engine component which reaches a temperature up to 2000 F.

The nickel base alloy and cobalt base alloy turbine buckets and other components commonly used today in gas turbine engines for aircraft normally have maximum service temperatures of approximately 1800 F. to 1900 F. This limitation necessarily restricts the performance and efficiency of these engines. Refractory metals, such as niobium, tungsten, molybdenum and chromium, have satisfactory high melting temperatures and suificient potential availability to warrant investigation as high temperature turbine blade materials. However, each of these metals exhibits poor oxidation resistance at temperatures of 2000 F. or above. Such metals are therefore unsatisfactory for use in turbine blades or other engine components which are exposed to extremely hot oxidizing gases. During recent years attempts have been made to correct this deficiency by adding various alloying elements to these refractory base metals. However, these attempts were generally unsuccessful since some of the resultant products still did not possess adequate oxidation resistance at the very high temperatures under consideration, while others were excessively brittle or weak.

Therefore, a principal object of the present invention is to provide a refractory alloy which can be employed as a material for turbine blades or fabricated components of turbine engines at temperatures up to 2000 F. because of its high hot strength, hot ductility and oxidation resistance at such elevated temperatures. It is considered desirable that an alloy to be used for gas turbine blade service in air at 2000 F. have at least a IOU-hour stressrupture life at that temperature under a load of 15,000 psi. Such an alloy must possess the necessary fabricability, room temperature impact strength and a melting point of at least 3000 F.

In accordance with my invention, 1 have found that the foregoing requirements are satisfied to an outstanding degree by a refractory alloy comprising about 9% to 11% molybdenum, 3% to 7% titanium, to tungsten and the balance (74% to 83%) substantially all niobium. A small amount of zirconium, preferably about 0.5% to 2%, may be included in the alloy to further increase its strength Without adversely affecting its extreme high temperature oxidation resistance. Additional advantages also are provided with respect to particular mechanical properties when the alloy contains hafnium and aluminum in amounts not exceeding approximately 10% and 5%, respectively.

The molybdenum serves to materially increase the 3,125,445 Patented Mar. 17, lilfi i ice oxidation resistance of the niobium base alloy and also contributes to its hot strength. When the molybdenum content is lower than about 9% or higher than approx mately 11%, the oxidation resistance of the alloy is noticeably reduced.

Titanium further improves resistance to high temperature oxidation, although it is also necessary in order to provide the niobium base alloy with the desired amount of ductility. If the amount of titanium present is less than about 3%, the oxidation resistance of the alloy is inadequate and the alloy tends to become brittle. On the other hand, when more than 7% titanium is present, the alloy does not possess sufficient strength and hardness for the applicaitons under consideration. In general, a 3% to 5% titanium content is preferred.

The tungsten is added principally to improve the hot strength of the niobium base alloy, although it also increases its oxidation resistance. A tungsten content of at least 5% is necessary for sufficient oxidation resistance, but if more than about 10% tungsten is added the oxidation resistance likewise is reduced to an undesirable level. The addition of approximately 7% to 8% tungsten is preferred to achieve maximum oxidation resistance.

When hafnium is included in the niobium base alloy it functions primarily to improve the hot hardness and hot tensile strength of the alloy. Hafnium also increases the oxidation resistance of the alloy to some extent. If more than 10% hafnium is added, however, the alloy undergoes a change in microstructure with temperature variations and become metallurgically unstable with resultant embrittlement. Optimum oxidation resistance appears to be obtainable when the alloy contains about 5% to 10% hafnium.

Zirconium serves to further increase the strength of the niobium base alloy, but if more than approximately 2% zirconium is present, the alloy becomes susce tibie to atmospheric contamination to an excessive extent. In general, an amount of zirconium as small as 0.5% provides a detectable increase in hot strength.

Up to 5% aluminum slightly improves the oxidation resistance of the niobium base alloy without reducing its hot strength. If the aluminum content is raised above this level, however, the ductility of the alloy is reduced because the excess aluminum appears to promote the formation of brittle inter-metallic compounds. An aluminum addition of less than about 0.5% does not appear to materially increase oxidation resistance. Small amounts of various other elements, usually not more than 2% or 3%, can be included in the alloy without detracting from its mechanical properties.

The above-described niobium base alloy has been prepared by alloying high purity elemental raw materials in an inert atmosphere of argon plus helium. It was found that the constituents of the alloy may be added either simultaneously or successively. Melting was accomplished with a nonconsumable tungsten electrode electric arc. After solidification of the alloy, it was crushed to fine particle size and re-melted in the foregoing maner and cast in ingot form. These ingots were then enclosed by welding in a container formed of an alloy of molybdenum plus 0.5% titanium, heated to a temperature of approximately 2850 F. optical pyrometer reading, and extruded to approximately reduction in cross-sectional area. One-half inch diameter bar stock 3 was produced with the niobium base alloy of this invention by this direct extrusion of cast ingots, the extruded alloy having a fibrous appearing cold worked structure. Subsequent further working has been readily accomplished by both swaging and rolling. For example, a sheet having a thickness of 0.01 inch with approximately 95% cold work was produced by rolling a portion of the bar stock at 600 F. When this sheet material was subjected to a stress-relief anneal at a temperature of about 2000 F., it could be bent 90 around a one-eighth inch diameter anvil at room temperature without cracking.

Test bars were machined from the extruded niobium base alloy bar stock and stress-rupture tested in a vacuum at a temperature of 2000 F. under high loads to evaluate the hot strength and hot ductility of the alloy. In one instance a test bar was successively subjected to a 15000 p.s.i. load for 100 hours and a 20,000 p.s.i. load for 30.2 hours before rupture occurred. The residual elongation at rupture was 47.5% with reduction in area of 57.5%. Another test bar, which was subjected to a load of 20,000 p.s.i. at the same temperature under vacuum, required 48.5 hours to rupture after undergoing a 73.4% reduction in area and a 80.4% elongation. These test bars were formed of a niobium base alloy composed of 75% niobium, 10% molybdenum, 7.5% tungsten, 5% titanium and 2.5 aluminum.

The typical commercially available nickel base alloys which are sufliciently ductile to be used in sheet form have materially less strength than the alloy of this invention. For example, a commercial alloy consisting of 0.04% carbon, 0.5% manganese, 7% iron, 14.5% chromium, 0.7% aluminum, 2.5% titanium, 1% niobium plus tantalum and the balance nickel, has a 10-hour stressrupture life under a load of only 4,000 p.s.i. at 1800 F. Hence the hot strength of my new alloy compares favorably with the hot strength of sheet materials heretofore employed in high temperature applications. The new alloy thus may be beneficially used for turbine blades and other turbine engine components.

Tests also were conducted to determine the room temperature impact strength of this niobium base alloy. In making this evaluation, an ingot having a diameter of two inches and a length of two inches was cast and extruded in the above-described manner. A billet from the extruded bar stock was recrystallized to a fine grain size at a temperature of 2550 F., canned in a stainless steel container, swaged to 50% reduction in cross section at 2000 F. and finished to 75% total reduction at 1800 F. Impact bars, each having a length of one inch and a width and thickness of one-quarter inch, were then machined from the swaged bar stock and were found to have impact strengths of from 14.6 foot-pounds to more than 15 foot-pounds. The recrystalilzation temperature of my new alloy was found to be approximately 2450" F. when cold worked to a 5 reduction in cross section.

I have likewise evaluated the niobium base alloy of this invention from the standpoint of oxidation resistance. For example, an alloy consisting of approximately 75% niobium, 10% molybdenum, 7.5% tungsten, titanium and 2.5% aluminum had a total surface metal loss of less than 5 mils in thickness after 100 hours cyclic exposure in air at a temperature of 2000 F. By comparison, pure niobium undergoes a surface metal loss of approximately one-quarter inch when subjected to the same test, thus demonstrating the greatly improved oxidation resistance of my new alloy.

My tests also showed that the addition of a small amount of zirconium with a corresponding reduction in the titanium content resulted in an alloy having further improved hot hardness at a temperature of 2000 F. For example, an alloy composed of 75 niobium, molybdenum, 7.5% tungsten, 3% titanium, 2% zirconium and 2.5 aluminum exhibited a diamond pyramid hardness of 152 at that temperature compared with a 107 diamond pyramid hardness reading for the same alloy containing 5% titanium and no Zirconium. Similar increases in hot hardnesses were obtained when 5% to 10% hafnium was included in the base alloy at the expense of niobium. An alloy of 65% to 70% niobium, 10% molybednum, 7.5% tungsten, 5% titanium, 2.5% aluminum and 5% to 10% hafnium has shown hardness values as high as 165 diamond pyramid hardness at 2000 F. The hafnium also increases the oxidation resistance of the alloy to some extent by producing a more dense protective oxide surface layer.

Thus it will be seen that the niobium base alloy of the present invention possesses a combination of high hot strength, fabricability, high room temperature impact strength, excellent hot ductility and good oxidation resistance at extreme temperatures. Moreover, the alloy has a melting point materially in excess of 3000 F., the desired minimum temperature hereinbefore mentioned. For example, the alloy consisting of 75 niobium, 10% molybdenum, 7.5 tungsten, 5% titanium and 2.5 aluminum possesses a melting point of approximately 4200" F.

While my invention has been described by means of certain specific examples, it is to be understood that the scope of the inevntion is not to be limited thereby except as defined by the following claims.

I claim:

1. A high temperature niobium base alloy having an outstanding combination of fabricability, hot ductility, oxidation resistance and stress-rupture life of more than hours under a load of 15,000 p.s.i. at a temperature of 2000 F., said alloy comprising about 9% to 11% molybdenum, 3% to 7% titanium, 5% to 10% aluminum, 0.5% to 2% zirconium, 5% to 10% hafnium and the balance substantially all niobium.

2. A high temperature niobium base alloy having an outstanding combination of fabricability, hot ductility, oxidation resistance and stress-rupture life of more than 100 hours under a load of 15,000 p.s.i. at a temperature of 2000 F., said alloy comprising about 9% to 11% molybdenum, 3% to 7% titanium, 5% to 10% tungsten, 5% to 10% hafnium, 0.5% to 5% aluminum and the balance substantially all niobium.

3. A high temperature niobium base alloy having an outstanding combination of fabricability, hot ductility, oxidation resistance and stress-rupture life of more than 100 hours under a load of 15,000 p.s.i. at a temperature of 2000 F., said alloy comprising about 9% to 11% molybdenum, 3% to 7% titanium, 5% to 10% tungsten, 0.5 to 2% zirconium, 0.5 to 5% aluminum and the balance substantially all niobium.

4. An oxidation-resistant, high temperature niobium base alloy comprising about 9% to 11% molybdenum, 7% to 8% tungsten, 0.5% to 2% zirconium, 5% to 10% hafnium, 3% to 5% titanium and the balance substantially all niobium.

5. A high temperature niobium base alloy having an outstanding combination of fabricability, hot ductility, oxidation resistance and stress-rupture life of more than 100 hours under a load of 15,000 p.s.i. at a temperature of 2000 F., said alloy comprising about 9% to 11% molybdenum, 3% to 7% titanium, 5% to 10% tungsten, 0.5 to 2% zirconium and the balance substantially all niobium.

6. A high temperature niobium base alloy having an outstanding combination of fabricability, hot ductility, oxidation resistance and stress-rupture life of more than 100 hours under a load of 15,000 p.s.i. at a temperature of 2000 F., said alloy comprising about 9% to 11% molybdenum, 3% to 7% titanium, 5% to 10% tungsten, 5% to 10% hafnium and the balance substantially all niobium.

7. A gas turbine blade characterized by high hot strength, excellent hot ductility and oxidation resistance upon exposure to oxidizing gases at a temperature of 2000 F., said blade being formed of an alloy comprising about 9% to 11% molybdenum, 0.5% to 2% zirconi hafnium not in excess of 10%, 3% to 7% titanium, 5% to 10% tungsten, 0.5% to 5% aluminum and the balance substantially all niobium.

8. A blade for a gas turbine engine characterized by high hot strength, excellent fabricability, outstanding hot ductility and oxidation resistance upon exposure to oxidizing gases at a temperature of 2000 F., said blade being formed of an alloy consisting essentially of about 9% to 6 11% molybdenum, 3% to 7% titanium, 5% to 10% tungsten, 5% to 10% hafnium, zirconium not in excess of 2%, 0.5% to 5% aluminum and the balance substantially all niobium.

References Cited in the file of this patent UNITED STATES PATENTS 2,822,268 Hix Feb. 4, 1958 2,882,146 Rhodin Apr. 14, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 125,445 March 17, 1964 Neil M. Lottrijge, Jr.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 41, line 32, for "aluminum,

read tungsten 15% to 0% aluminum,

Signed and scaled ililiS 8th day oi September 1964.

(SEAL) A tlest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A HIGH TEMPERATURE NIOBIUM BASE ALLOY HAVING AN OUTSTANDING COMBINATIN OF FABRICABILITY, HOT DUCTILITY, OXIDATION RESISTANCE AND STRESS-RUPTURE LIFE OF MORE THAN 100 HOURS UNDER A LOAD OF 15,000 P.S.I. AT A TEMPERATURE OF 2000*F., SAID ALLOY COMPRISING ABOUT 9% TO 11% MOLYBDEMUN, 3% TO 7% TITANIUM, 5% TO 10% ALUMINUM, 0.5% TO 2% ZIRCONIUM, 5% TO 10% HAFNIUM AND THE BALANCE SUBSTANTIALLY ALL NIOBIUM. 